FIG. 2 is a cross-sectional view showing a structure of a quantum wire semiconductor laser device disclosed in a paper by E. Kapon, Applied Physics Letter 55(26), 25th December 1989, pp. 2715 to 2717. The device of FIG. 2 includes an n.sup.+ type GaAs substrate 1 patterned with a V-shaped stripe groove. An n type Al.sub.y Ga.sub.l-y As cladding layer 2, an Al.sub.x Ga.sub.l-x As first SCH (Separate Confinement Heterostructure) layer 3, a GaAs quantum well layer 4, an Al.sub.x Ga.sub.l-x As second SCH layer 5, a p type Al.sub.y Ga.sub.l-y As cladding layer 6, and a p.sup.+ type GaAs contact layer 7 are successively laminated on the substrate 1 keeping the configuration of the V-shaped stripe groove. A p side electrode 8 is disposed on the contact layer 7 and an n side electrode (not shown here) is disposed on the entire rear surface of the substrate 1. A high resistance region 22 is formed in the p type Al.sub.y Ga.sub.l-y As cladding layer 6 and the p.sup.+ type GaAs contact layer 7 by proton implantation
A description is given of the operation.
When a current at least equal to the threshold current or more is injected in the forward direction to the pn junction of the quantum wire semiconductor laser shown in FIG. 2, laser oscillation occurs in the well layer 4 and laser light is emitted. While an ordinary semiconductor laser has an active region of approximately 0.05 to 0.2 micron thickness, a quantum well laser generally has a quantum well layer of no more than approximately 300 angstroms thickness. In such a thin layer, a quantum effect occurs so that electrons are localized in the film thickness direction. As a result, a higher gain is obtained in the quantum well laser than in the ordinary semiconductor laser and reductions in the threshold current and the operation current are expected. The quantum wire semiconductor laser is obtained because that quantization is also realized in the horizontal direction in addition to the layer thickness direction. This quantum wire semiconductor laser can present more significant quantization effects.
In the quantum wire semiconductor laser device shown in FIG. 2, a potential barrier is also formed in the horizontal direction by the epitaxial growth layers on the V-shaped groove. Electrons and holes are confined in the potential barrier and then quantized. In addition, when the length of the quantum wire stripe is no more than approximately 500 angstroms, a so-called quantum box in which electrons and holes are confined three-dimensionally is obtained, so that the quantum effect is more apparent.
In the device of FIG. 2, the width W.sub.z of the quantum wire in the horizontal direction largely depends on the configuration of the V-shaped groove and the rate of the epitaxial growth of the epitaxial growth layers. This makes it quite difficult to control the width W.sub.z precisely. Further, the maximum output power of a semiconductor laser is generally limited to a level at which catastrophic optical damage (hereinafter referred to as COD) of the facet occurs. In order to heighten the COD level to enhance the maximum output power, it is required to increase the cross-sectional area of the light emitting region. However, in the structure of FIG. 2, it is impossible to provide two or more quantum wires in the active region 1, making it impossible to obtain a high output power.
FIG. 3(a) is a cross-sectional view showing a semiconductor laser device utilizing a two-dimensional multi quantum well structure, disclosed in Japanese Published Patent Application No. 63-29989. The device of FIG. 3(a) has a p type GaAs substrate 31. A p type Al.sub.0.3 Ga.sub.O.7 As cladding layer 32 is disposed on the substrate 31. A two-dimensional multi-quantum well active layer 37 having stripe configuration, connecting the facets and constituting a resonator, is disposed on a center part of the cladding layer 32 relative to the width of the laser device. A silicon dioxide (SiO.sub.2) insulating film 38 is disposed on the cladding layer 32 and the side walls of the active layer 37. An n type Al.sub.0.3 Ga.sub.0.7 As cladding layer 39 is disposed on the insulating film 38 and the active layer 37. An n type GaAs contact layer 40 is disposed on the cladding layer 39. A p side electrode 42 is disposed on the rear surface of the substrate 31 and an n side electrode 41 is disposed on the contact layer 40.
A description is given of the structure and production process of the two-dimensional multi-quantum well active layer 37 of this prior art laser device. FIGS. 3(b) and 3(c) show the production process of the two-dimensional multi-quantum well active layer 37 shown in FIG. 3(a).
First, cladding layer 32 is epitaxially grown on the substrate 31. Then, an AlGaAs layer 33A of 50 angstroms thickness and a GaAs layer 33B of 50 angstroms thickness are alternatingly laminated ten times on cladding layer 32 to form a laminated structure. Thereafter, photoresist film 34 is patterned on the laminated structure, and then the laminated structure comprising AlGaAs layers 33A and GaAs layers 33B is etched away using photoresist film 34 as a mask. Then, the side surface of the remaining laminated structure is further etched away by a reactive ion etching to form periodic concave parts 33C each having a depth 1 as shown in FIG. 3(b). Here, the depth 1 is 50 angstroms. Such an etching configuration can be realized because it is possible in reactive ion etching to set the etching rate of AlGaAs about 200 times as high as that of GaAs by setting the etching conditions appropriately.
Next, on the side surface of the laminated structure having periodic concave parts 33C, a GaAs film 37B and an AlGaAs film 37A are alternatively grown by a vapor phase epitaxy. By using the vapor phase epitaxy method, respective films are grown on the side surface of the laminated structure reproducing the concavo-convex configuration thereof precisely, as shown in FIG. 3(c). The alternating growths of GaAs film 37B and AlGaAs film 37A are repeated until the width of active region 37 becomes approximately 0.8 to 1 micron, resulting in the structure shown in FIG. 3(c).
Thereafter, the laminated structure comprising AlGaAs layer 33A and GaAs layer 33B is etched away by a usual photolithography technique and a dry etching so as to form active region 37 in a stripe configuration Then, insulating film 38, cladding layer 39 and contact layer 40 are formed thereon and the electrodes 41 and 42 are formed on the contact layer 40 and on the rear surface of substrate 31, respectively. Thus, the laser structure shown in FIG. 3(a) is completed.
A description is given of the operation hereinafter.
In the device of FIG. 3(a), when a voltage is applied across the electrodes 41 and 42 in a forward direction with respect to the pn junction, carriers are injected into active layer 37 and then confined in a region having a small energy band gap in active layer 37, i.e., in GaAs film 37B shown in FIG. 3(c), and recombine therein, thereby emitting light. The emitted light is reflected and amplified between the cleavage facets provided opposite to each other and perpendicular to the active layer stripe, thereby producing laser oscillation. Here, since GaAs film 37B has a very narrow and slender linear configuration whose cross-sectional dimension is approximately 50 angstroms along each edge, superior laser characteristics, i.e., reduced current, due to the effect of the quantization of injected carriers can be obtained. In addition, in this prior art structure, it is possible to form a plurality of quantum wires in the active layer, so that a laser device having a high output power can be realized.
In the prior art quantum wire semiconductor laser device constituted as described above, several etching processes are required for forming the active layer and, therefore, the production process is quite complicated. In addition, it is quite difficult to apply the prior art quantum wire structure to a general laser device such as a ridge type laser device or a groove type laser device.